1. Field of the Invention
The present invention relates to a technique to moderate an electric field concentration caused by a conductive film which is formed on a surface of a semiconductor device.
2. Description of the Background Art
Normally, doped regions to be connected are exposed at a surface of a semiconductor device. A conductive film is provided, with an insulation layer intervened, on the surface of the semiconductor device to connect the doped regions.
FIG. 1 is a cross sectional view of a conventional semiconductor device. A p type diffusion region 2 and an n type diffusion region 3 are formed in space in a surface of an n.sup.- type layer 1. In the surface of an n.sup.- type layer 1, the p type diffusion region 2 and the n type diffusion region 3 are separated by an insulation layer 6b. A conductive film 5a is in contact with the diffusion region 2 but otherwise insulated by an insulation layer 6a and the insulation layer 6b. A conductive film 5e is in contact with the diffusion region 3 but otherwise insulated by the insulation layers 6a and 6b. The conductive films 5a and 5e and still other conductive films 5b, 5c and 5d are vertically insulated from each other by the interleaved insulation layers 6a and 6b but horizontally overlap each other. Hence, adjacent conductive films are coupled to each other.
A conductive film 4 is connected with the diffusion region 3 and extends from the diffusion region 3 over the diffusion region 2. The interposed insulation layer 6a insulates the conductive film 4 from the other regions. The conductive film 4 is coupled to the conductive films 5a to 5d.
With a low voltage (-V) applied to the diffusion region 2 and a high voltage (+V) applied to the diffusion region 3, a depletion layer extends from a junction J1 between the n.sup.- type layer 1 and the diffusion region 2. In the surface of the n.sup.- type layer 1 (i.e., immediately below the insulation layer 6b), the depletion layer extends from the diffusion region 2 toward the diffusion region 3.
The device structure as above places the growth of the depletion layer at the surface of the n.sup.- type layer 1 under the influence of a potential at the conductive film 4. Being at the same level as a potential at the diffusion region 3, a potential at the conductive film 4 restrains the growth of the depletion layer, with a result that an electric field concentration is created near the diffusion region 2. During the presence of the electric field concentration, it is difficult to obtain enough breakdown voltage. On the other hand, when the conductive film 4 is connected with the diffusion region 2 and therefore has the same potential as the diffusion region 2, the conductive film 4 helps the depletion growth, whereby an electric field concentration occurs near the diffusion region 3.
The influence of a potential at the conductive film 4 is eliminated by disposing the conductive films so that the conductive films 5a to 5e each have a negligible capacitance with the conductive film 4 as compared to a neighboring conductive film. When this approach is adopted, optimum potentials are obtainable at the conductive films 5a to 5e under almost no influence of a potential developed at the conductive film 4. Thus, a potential at the conductive film 4 exerts only negligible influence over the depletion layer growth in the surface of the n.sup.- type layer 1. Hence, deterioration in the breakdown voltage due to an electrical field from the conductive film 4 is prevented.
Despite the advantage as above, the conventional semiconductor device is not satisfactory in terms of process simplicity since the conductive films 5a to 5e need to overlap each other to ensure sufficient capacitances therebetween. To obtain such a stacked arrangement, the conductive films 5a to 5e must be formed in two separate process steps.